A laser beam can be focussed on an area equal to the square of its wavelength A $He-Ne$  laser radiates energy at the rate of $1\,mW$ and its wavelength is $632.8 \,nm$. The intensity of focussed beam will be

A linearly polarized electromagnetic wave in vacuum is $E=3.1 \cos \left[(1.8) z-\left(5.4 \times 10^{6}\right) {t}\right] \hat{\text { i }}\, {N} / {C}$ is incident normally on a perfectly reflecting wall at $z=a$. Choose the correct option

The intensity of light from a source is $\left( {\frac{{500}}{\pi }} \right)W/{m^2}$ . Find the amplitude of electric field in this wave

In a certain region uniform electric field $E$ and magnetic field $B$ are present in the opposite direction. At the instant $t = 0,$ a particle of mass $m$ carrying a charge $q$ is given velocity $v_o$ at an angle $\theta ,$ with the $y$ axis, in the $yz$ plane. The time after which the speed of the particle would be minimum is equal to

The electric field part of an electromagnetic wave in a medium is represented by $E_x = 0\,;$

${E_y} = 2.5\,\frac{N}{C}\,\,\cos \,\left[ {\left( {2\pi \, \times \,{{10}^6}\,\frac{{rad}}{m}} \right)t – \left( {\pi  \times {{10}^{ – 2}}\frac{{rad}}{s}} \right)x} \right];$

$E_z = 0$. The wave is